Aluminum alloy



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1., mum-4g a0 50 mo I I 200 ARTIFICIAL AGEING TEMPERATURE c 1 MM MpMPatented Aug 8, 1 933 ALLOY Karl Leo Meissner, Duren, Germany, assigncrto Durener Metallwerke Aktiengesellschaft, Duren, Germany, a Germanjoint stock Com- Application March 30, 1931, Serial No. 526,525,

- and in Germany April 1, 1930 3- Claims.

It is known that further increases in the yield point beyond the valuesobtained by the natural aging of self tempering aluminum alloys may beobtained in the following way:

5 1. By the generally known process of cold working which is applicablenot only to aluminum alloys that may be tempered but quite generally toall practically "deformable metals and alloys.

2. By the use of theso-called artificial aging, i. e., a heat hardeningprocess carried out at moderately high temperatures. Such a process isdisclosed, for example, in the French Specification No. 555,836,Leichtmetall Studien 11nd Ver.- wertungs Gesellschaft m. b. H. of April3, 1923, or in the United States Patent No. 1,695,044 to Hallmann,granted December 11, 1928, there being, however, as compared with theprocess ac cording to the present invention, a difference inasmuch as inthe first place, cold working is employed not before natural aging iscomplete but at an earlier point, and also in that the tempera-' 3 ityof the effect of artificial aging that the falling off in theaforementioned properties beginsjust at the moment when the yield pointcommences to rise. For these reasons, a technical gain cannot be notedeither with the one or the other of the known processes, because theonesided improvement of the alloy can only be-secured at the expense ofseveral other likewise valuable mechanical and chemical properties.

A process is also known which is chiefly employed in the manufacture ofwire of high electrical conductivity from aluminum alloys and whereinthe rolled or extruded wire is heated to high temperatures, is quenchedand is thereupon subjected, without long natural aging, to a 50 veryextensive cold working by drawing, the wire being thereupon annealed ata temperature below 250 C; until the desired conductivity is attained.This process differs iron; that according to the present invention bythe very important feature that no storing is provided after quenchingand nesium and 0.75% silicon, after quenching from moreover also in thatthe cold working by drawing is very much greater than in the presentprocess, being namely about 95%..

A process relating to the same wire alloy for overhead conductors isemployed to increase the yield point and elastic limit, whereby thefinished drawn wire is stretched, after annealing, vby 0.5 to 2% and isthen re-annealed at 50-160 C. This. process likewise differs in thefollowing way from that according to the present invention.

Due to the pretreatment described in the foregoing which departsentirelyfrom that of the present invention, the wire is in quite adifferent condition from the outset, and consequently also, in thestretching of the wire, thesubsequent anhealing treatment causes 'afurther rise in the yield point and elastic limit, whereas in theprocess according tothe present invention, it is just'the reverse, theannealing treatment being carried out within such a temperature rangethat scribed, inter alia, a test on duralumin, whereby the material washeated, quenched and then subjected to cold working. This test,howeverxlacked the most important feature of the present invention,namely, the subsequent annealing at such temperatures and for suchlengths of time that the elongation lowered by the cold working israised again to the value of the naturally aged alloy while preserving,at the same time, an in-, creased yield point.

In the United States Patent No. 1,472,739, granted October 30, 1923 toR. S. Archer andZ. Jeifries, it is stated that an alloy of 0.5% mag- 550C. was stretched by 15% 'in a machine for testing tensile strength andwas thereupon aged artificially for .60 hours at C. -.After thistreatment the alloy exhibited certain properties an elongation of 37.5%.

differs from thepresent invention according to which an important partis played bythe sequence of the individual steps, and moreparticupresent invention-results in its success. At this hightemperature, combined with such a long space of time, a pronouncedeffect of artificial aging is concerned, and this term is also expresslyemployed in thespecification as follows also from the elongation of12.5%, which is remarkably low for such a slightly alloyed composition.In the naturally aged condition, however, the alloy has Acording to thepresent invention, the procedure is such that the elongation is raisedagain to the normal value, whereas it followsfrom the statement made inthe United States specification that the elongation in the treatmentdescribed falls to of the normal value. a r

In contradistinction to the known processes described in the foregoing,it is the object of the present invention to attain an increase, in theyield point with simultaneous moderate increase in the tensile strength,without at the same time] having any detrimental effect on theelongation and on the other properties mentioned in the foregoing,namely, deformability and resistance to corrosion. The two exampleshereunder will serve to explain the essential features ofthe invention.

An alloy with 4.2% copper, 0.5% magnesium, 0.25% manganese and about0.25-0.3% each of iron and silicon as impurities gave in the improved orheat treated condition a yield point of about 25 ,kg/mm a tensilestrength of about 40-42 kg/mm 20-22%. When this alloy, after naturalaging was further annealed at high temperatures, i.'e.

was subjected to theprocess of artificial aging, a

drop in the yield point and also in the tensile strength, attaining aminimum value with an aging period of 20 to 40 hours at about 100 C.,was observed in the temperature ranges at which no diminution in theelongation and ductility couldyet be detected. The yield point fell toabout 21-22, and the tensile strength to about" 37-38 kgJmm. Thebehavior of the yield point in this experiment may be gathered from thedotted line curves in Figure 1. p

The case was somewhat the same when the alloy was subjected to thesametreatment immediately after quenching from the high temperaturetreatment. Here again a drop in the yieldpoint and tensile strengthwasobserved, the yield point at IOU-125 C. falling to about 22-24, and thetensile strength to about 38-39 kg/mm. The behavior of the yield pointin this series-of experiments will be seen from thechain line curve inFigure 1. Both sets of curves substantially agreewith one another.

Thus, the annealing treatment, in the temperpoint values of 30-33 kg/mmwere obtained (see and an elongation of about point and tensile strengthin these temperature regions.

However, when still higher temperatures were employed at which the yieldpoint and also-to some extent the tensile strength increase, it islikewise not possible to attain a technical advantage, as alreadyexplained in the foregoing, because the elongation and deformability arelikewise unfavorably affected'by the same treatment.

Thesame alloy, after completion of natural aging, was now cold worked,the yield point being reduced to about 37-38 and the tensile strength toabout 46-47- kg/mm while the elongation fell in this treatment to about15-17%. On employing artificial aging on an alloy pretreated in thismanner, it was unexpectedly found that, on employingthe same agingperiod of20 to 40 hours, the yield point and tensile strength fell,reaching a minimum at about the same temperatures of -125 C., but thatthe drop only wentso far as such values as lie about 40% and more abovethose obtainedat the same temperatures in the two instances mentioned inthe foregoingfi In addition, these minimum values lay about 100 20-30%higher than the values exhibited by the yield point of the merelyimproved alloy. In the. temperature ranges of 100-125" (3., yield fullline curve in Figure 1). At the same time,

also lay about 5% higher than in the ,inerelyimprove'd state and about10-15% higher than the corresponding minimum values which were obtainedat the same temperatures with inclusion of 1 0 the cold working. At thesame time, the elongation, which had been reduced by cold working, roseagain to about 21% (see Figure'2) A magnesium free aluminum alloywithabout 5.8% copper, 0 .'7% manganese and again the usual impurities inthe form of iron and silicon had, in the improved condition, a yieldpoint of 27.4 kg/mm, a tensile'strength of 40.8 kg/mm' and 1 anelongation of16.2%. Since this alloy,

the tensile strength was 42-44 kg/mm, i. c. it

as compared with magnesium-containing alloy 13 mentioned in the firstexample, did not the feature of self-improving on standing at the roomtemperature, the alloy, after being quenched fromhigh temperaturetreatment, was further tempered for 24 hours at 145 C; In this conditionit was approximately comparable with the naturally aged condition of amagnesium-con--' tainingalloy. The alloy so treated having theabove-mentioned properties was now cold-worked and then annealed 20hours at'110. C. The yield point was then 33.4 kg/mm the tensilestrength 43.1 kg/mm while the elongation had again gone up to 16.5%. i

It may be stated with respect to sium content that in these.age-hardenable a1 loys amounts ofmagnesiuminexcess of, 2% are generallyrecognized as affordingno advantage, and thus when speaking of anage-hardenable alloy it is understood that the magnesium content doesnot exceed this limit. Thus in the Zeitchrift fur Flugtechnik undMotorluftschiffahrt, vol. 1'7, 1926, page 114, it is explained that themagnesium content of duralumin, a recognized age-hardenable aluminumalloy, should not in practice exceed 2%, since otherwise theage-hardening effect becomes smaller;

It is already known that the effect of cold working may be removed againby subsequent annealing and that the alloy may be made soft again, theproperties of strength falling and the the maK ej elongation anddeformability rising. For the alloys in question. this region liesgenerally at temperatures of about 350-400 C. The process according tothe present invention differs, however, from this generally knownannealing process in that, in the present process, it is not a questionof a purely annealing effect but of a peculiar combination of annealingtreatment and of an effect of artificial aging. which certainly onlyoccurs in a very narrow temperature range, which in the examplesdescribed in the foregoing lay at about 125'' C.

The effect of this combination in the narrow temperature range inquestion is that the elongation, as-in annealing, again rises to itsnormal original values, while obviously in the case of the yield pointand also the tensile strength, the effect of the annealing is partlycovered by the artificial aging. Since at the same time and at the sametemperatures. the deformability and the elongation both attain theirmaximum values, and a renewed falling ofi iii-these properties onlyoccurs when the above-mentioned temperature limit is exceeded, theapplication of the process depicted, in the individual steps asdescribed in the examples, provides on the whole a technical advantage,because a higher yieldpoint with moderately increased tensile strengthis now imparted to the material while preserving, however, the highvalues for the elongation and deformability, such as could not beimparted to it in this combination and at the same time in the processesknown heretofore.

I claim:--

1. In the tempering of age-hardenable aluminum alloys containingmagnesium in an amount. not exceeding 2%, the process of increasing theelongation while preserving at the same time a high yield pointconsisting in annealing for twenty to forty hours at a temperature ofabout 100"-125 C. after the alloy has been heated to the hightemperature, quenched. completely aged at room temperature and coldworked.

2. In the tempering of age-hardenable aluminum alloys containing about0.5% magnesium and about 4.2% copper, the process of increasing theelongation while preserving at the same time a high yield pointconsisting in annealing for 20 to 40 hours at a temperature of about100-125 C. after the alloy has been heated to the high temperature,quenched, completely aged at room temperature and cold worked.

3. In the tempering of age-hardenable aluminum alloys containing about0.5% magnesium, about .25% of manganese, and about 4.2% copper, theprocess of increasing the elongation lud while preserving at the sametime a high yield point consisting in annealing for 20 to 40 hours at atemperature of about 100 125" C. after the alloy has been heated to thehigh temperature. quenched, completely aged at room temperature 10.3

and cold worked.

KARL LEO MEISSNER.

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